A lawyer on trial on charges she let a suspect in the slaying of a Chicago police officer use her cellphone in an interrogation room testified Wednesday that she had no intention of obstructing the investigation and didn't know it was against the law to bring the phone with her.

Taking the stand in her own defense, Sladjana Vuckovic, who at the time was volunteering for a free legal service for indigent suspects, said she had made at least 100 client visits at police stations and had never been told that cellphones were prohibited.

Vuckovic, who is charged with a felony count of bringing contraband into a penal institution, faces up to 15 years in prison if convicted and would likely be disbarred.

The unusual nature of the charge has drawn a curious audience of defense attorneys, prosecutors and judges, who packed Judge Evelyn Clay's courtroom Wednesday to hear Vuckovic testify.

"I think I'm the first case ever in the country, pretty sure anyway," Vuckovic testified with a slight smile when her lawyer asked if she had ever heard of a colleague being accused of a similar crime.

Vuckovic, 44, told jurors she was vaguely aware of a state law prohibiting bringing contraband into a penal institution but thought it barred "knives, guns, drugs." She also said that unlike a police lockup where suspects are behind bars, she didn't consider an interrogation room to be a penal institution.

She told jurors she went to the Calumet Area headquarters in November 2010 to meet with Timothy Herring after a relative of his had contacted the legal hotline about his arrest. Herring was undergoing questioning by detectives in the slayings of Officer Michael Flisk and another man.

Vuckovic said she was there to provide a crucial service to Herring — to make sure he understood he had a constitutional right to have an attorney present during questioning and to refuse to cooperate with investigators if he so chose.

She said that while she was alone in the locked interrogation room with Herring, she allowed him to use her phone to talk to relatives but warned him it "was not the time" to discuss anything about the investigation.

"I told him, 'Call your family and friends, let them know you're OK, but do not talk about the case,'" Vuckovic testified. "'We have to just sit tight for 48 hours and see if they charge you or let you go.'"

Prosecutors alleged that 26 calls were made from or received by Vuckovic's cellphone during two meetings with Herring, including incoming calls from a number that was blocked.

The police lieutenant who discovered Herring on the phone testified Tuesday that he never would have allowed a suspect to make unmonitored calls because "he could compromise the entire investigation."

Vuckovic testified that most of the calls in the interrogation room were either dropped or didn't go through. The only conversations she could remember were with Herring's half-brother, Leonard, and those did not delve into any details of the investigation, she said.

During an often testy cross-examination, Cook County Assistant State's Attorney Thomas Mahoney wrangled with Vuckovic over her understanding of the terms "penal institution" and "contraband."

Mahoney also pressed Vuckovic on why she believed it was all right to have Herring make calls when he was chained to a ring in the wall and clearly in custody. A Chicago Police Department order states that a suspect should be allowed a "reasonable amount" of calls to family, friends or an attorney but that those calls could be monitored by police.

As far as the police policy, Vuckovic testified, "theory is one thing, and practice is another."

"It's been my experience that the police never give the person who is under arrest a phone call until after they have been charged," Vuckovic said.